In general, portable devices, such as a cellular phone, a Personal Digital Assistant (PDA), a Portable Media Player (PMP), a Digital Multimedia Broadcasting (DMB) terminal, an MPEG-1 audio layer-3 (MP3) player and a notebook computer, cannot be kept plugged into a regular power source at home or in an office since they are used while users are moving. Accordingly, such a portable device is equipped with a disposable battery (i.e., a primary battery) or a rechargeable battery (i.e., a secondary battery). In the case of recharging electric power, supplied from a regular power source, to the battery or a battery pack of the portable device, a method of supplying the electric power to the portable device via power supply lines or power supply connectors has traditionally been used.
However, in the case where the electric power is supplied to the portable device via the connectors, an instant discharge may happen from the connectors due to a potential difference between the connectors of a charger and the battery when the charger and the battery are connected to or disconnected from each other.
The instantaneous discharge may cause impurities to accumulate on both the connectors, thus resulting in a fire hazard.
In addition, since the connectors are directly exposed to the air, moisture or dust adheres to the connectors, thereby causing a natural discharge. As a result, the lifetime and performance of the charger and the battery may deteriorate.
In order to overcome problems related to the method of supplying electric power via the connectors, non-contact chargers have been developed. When a terminal containing a battery to be charged is placed on the primary coil of a charger, a secondary coil of a battery is charged. In this case, electric power is charged by inducing an electromotive force on the secondary coil using a magnetic field generated from the primary coil.
However, the non-contact charger of the related art can merely supply electric power to the portable device and its use is limited since it is not used for other applications.
The magnetic field generated from the primary coil changes when a metal body is placed inside it. Then, a considerable amount of power loss, which may damage the non-contact charger, occurs in the primary coil.
Since an excessive current flowing through the secondary coil and the circuit of the battery pack generates heat, the battery pack may explode due to excessive heat.
Furthermore, since most of the primary and secondary cores are implemented with one thick coil (hereinafter, referred to as a “Litz coil”) formed by twisting a number of fine wires, a great amount of coil materials are consumed and the charger becomes bulky. The charger then has a complicated structure and is difficult to manufacture. Mounting the charger on the portable device is often difficult because the portable device contains a number of semiconductor elements and the battery pack. Moreover, heat or a magnetic field may interfere with a plurality of controllers and members, thereby causing them to malfunction.
In addition, the charger is not applicable to a variety of technical fields since it is not easily manufactured in a variety of shapes. For example, one battery pack is not commonly used since the battery pack of the portable device such as a portable phone, a PDA, a PMP, a DMB terminal, an MP3 player, and a notebook computer has connectors installed in different places according to applications and manufacturers.
Since the non-contact charger of the related art also uses a thick coil, the charger is bulky, has a complicated structure, and is difficult to manufacture.
Moreover, the non-contact charger has limited applications since it is not easily manufactured in a variety of shapes. If the battery pack moves while being charged, power transmission is not properly achieved and thus transmission efficiency degrades.